The present invention relates to filtering techniques, and more particularly to the implementation of a digital filtering technique.
The use of filtering techniques to process digital information is well-known. One class of filtering known in the industry is a Fractional Phase Delay Filter (FPDF) that is used in many different signal processing tasks. Examples of such tasks include digital beam forming, audio resampling, image/video resizing, and modem front end (baud rate synchronization). In a specific example, FPDF filters are used in high-speed modems to eliminate analog voltage control oscillator (VCO) sample rate control circuitry. By replacing the analog portion of the VCO control circuitry with an FPDF filter, the resulting design is less sensitive to operating environment variations, such as voltage, temperature, and noise.
Traditionally, FPDF filters have been implemented using Finite Impulse Response (FIR) filters. FIR filters are commonly used to implement digital filters on semiconductor devices. Generally, FIR filters include a plurality of taps wherein receive coefficient data is used to multiply an input value at the specific tap. For example, a 7-tap FIR filter would perform 7 multiplications for each new input value received.
When FIR filters are used to implement poly-phase filters, each phase can have a different set of coefficients. Therefore, either an index table of coefficient values needs to be maintained, or a method of generating coefficient values as needed is used. The number of phases in a system is dependent upon the design specification. However, for audio applications, the number of required phases can be tens-of thousands. When each phase requires plurality of coefficients, each of which can be two or more bytes in length, the storage space needed to save the coefficients, or the hardware needed to generate them is a significant resource in terms of the system area needed to store the values. In addition, generic multipliers are generally needed with such system to be able to handle the range of coefficient values.
Therefore, the amount of area required to implement poly-phase filters using FIR filters can be costly. Thus, a method and/or apparatus capable of implementing filtering techniques, such as FPDF filters, without the overhead associated with multiplication units and coefficient storage or generation would be useful.